The invention relates to a method of producing amorphous carbon coatings on substrates by the degradation of a gaseous carbon compound in an ionized gaseous atmosphere within a reaction chamber, using an alternating electromagnetic field to excite the plasma.
The term "amorphous" applied to carbon coatings originates from initial studies indicating a largely amorphous structure. In the meantime, later studies have proven that a typical diamantine bond is also present within small areas. The term is, therefore, one of art and not rigor.
At the present time, the following is known concerning the basic conditions under which an amorphous carbon coating can be produced: the energy of the carbon ions or hydrocarbon radicals impinging upon the substrate must exceed a certain threshold value; approximately 30 atomic percent of hydrogen can be incorporated in the coating, and, depending on the conditions of formation, different sub-nomenclatures have become established in the art for the coatings, viz:
amorphous carbon PA1 i-carbon (i represents "ion assisted") PA1 a-C:H (hydrogenated amorphous carbon) PA1 diamond-like carbon. PA1 (a) In an initial step in the method, a gas from the group of the siloxanes or silazanes is introduced into the reaction chamber. PA1 (b) A ground coating of a polymer of the siloxanes or silazanes is formed on the substrates. PA1 (c) Then the gaseous hydrocarbon compound is introduced into the reaction chamber. PA1 (d) The amorphous carbon coating is formed on the ground coat.
The amorphous carbon coatings have great hardness. They are chemically inert and permeable to infrared radiation, so that there is a considerable demand for such coatings as mechanical and chemical protective coatings on a wide variety of substrates in common use, and as an optically active coating on special optical substrates permeable to infrared radiation.
German Offenlegungsschrift No. 1,736,514 and the corresponding British Pat. No. 1,582,231 discloses a process for coating a substrate with amorphous carbon in a reaction chamber in which the substrate with the substrate holder forms one plate of a capacitor to which an electrical frequency of between 0.5 and 100 MHz is applied to produce a coating plasma from a monomer gas in the reaction chamber. The other plate of the capacitor can be the floor of the reaction chamber, but it can also be formed by a second plate in the reaction chamber. In either case, both plates are in the reaction chamber and consequently also exposed to the ionized gaseous atmosphere, and this would have to be the case in any event with the plate which forms the substrate holder. Contamination of the coating with the plate material can result.
Experience has shown, moreover, that the known method and apparatus can only achieve rates of carbon deposition of between 1.0 and 3.0 nm/sec. It is also difficult to obtain sufficient uniformity of coating thickness, because variations in the coating thickness can be caused by an irregular energy input into the plasma, and differences in monomer concentration near the substrate. This is further complicated by the fact that gaseous reaction products are formed as the coating builds up, which must be continuously pumped out. By the presently known method, in the case of planar substrates of a diameter of only 20 cm, thickness irregularities of approximately 5% are achieved over the entire area. Evidently this is true of no more than laboratory-scale production, which is not easily, if at all, transferrable to large technical processes.
It is therefore an object of the invention to provide a method of the kind described above, whereby the rates of deposition can be considerably increased and substrates of large area can be uniformly coated.